2-(cyclohex-1′-enyl)cyclohexanone is an important, widely used chemical product. In addition to being used as a modifying agent for epoxy resins, a plasticizer, and a crosslinking agent for polymers, it is also an intermediate in the synthesis of o-phenylphenol, which is an important fine organic chemical. O-phenylphenol can be widely used in synthesizing dyes, surfactants, fire retardants, plastic stabilizing agents, pharmaceuticals, etc., and its market demand also increases continuously as its uses are developed unceasingly.
O-phenylphenol can be produced: 1. by dehydro-condensation of cyclohexanone, 2. by sulfonation- or halogenation-hydrolysis of phenylbenzene, 3. by diazotization-hydrolysis of aminophenylbenzene, 4. by hydrolysis of chlorobenzene under high pressure, 5. from dibenzofuran, or 6. by coupling of chlorobenzene and phenol. Among the aforementioned synthesis processes, the most widely used process is currently the process of dehydro-condensation of cyclohexanone. The process of dehydro-condensation of cyclohexanone can be divided into two stages; the first stage is condensing cyclohexanone in the presence of an acidic or basic catalyst to obtain a dimer of 2-(cyclohex-1′-enyl)cyclohexanone, and the second stage is dehydrogenating 2-(cyclohex-1′-enyl)cyclohexanone with a catalyst containing a metal such as Pt to produce o-phenylphenol.
As stated above, 2-(cyclohex-1′-enyl)cyclohexanone is produced from the acid- or base-catalyzed condensation occurring between cyclohexanone molecules, and its reaction equation is shown as below:

Generally, the auto-condensation of cyclohexanone will produce two kinds of resonance isomers (I) and (II), of which 2-(cyclohex-1′-enyl)cyclohexanone (II) is usually predominant. Both the two kinds of compounds can proceed to the subsequent dehydrogenation reaction to prepare o-phenylphenol.
Currently, the published patents relating to catalysts for use in the auto-condensation of cyclohexanone to produce 2-(cyclohex-1′-enyl)cyclohexanone can be roughly classified into four kinds, including inorganic strong acids/bases, organic acids, organic metal compounds, and solid acidic catalysts.
In U.S. Pat. No. 4,002,693, sulfuric acid is used as the catalyst, and the yield of 2-(cyclohex-1′-enyl)cyclohexanone is about 37%. However, the use of sulfuric acid as the catalyst has many disadvantages. First, the investment in the reactor equipment is higher due to the strong corrosive property of sulfuric acid. Second, if concentrated sulfuric acid is used as the catalyst, its strong oxidation property and strong dehydration property will cause increased reaction by-products; on the other hand, the use of diluted sulfuric acid may reduce the occurrence of side reactions, but will cause excessive water to remain in the reaction system, which will lower the convertibility of the reaction. Third, the catalyst of sulfuric acid cannot be reused, the cost of the process is high, and the waste water from the process causes environmental problems.
U.S. Pat. No. 3,980,716 discloses that a compound formed of aliphatic carboxylic acid, naphthenic acid, heteropolyacid, ethylenediamine tetra-acetic acid and acetylacetonate with a metal such as Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Mo, Cd, Sn and W, or their alkoxides can be used as the catalyst in the auto-condensation of cyclohexanone to produce dimers, in which vanadium stearate as the catalyst has a better performance, and the conversion of cyclohexanone and the selectivity of dimer are 71% and 89%, respectively. However, the reaction must be carried out in the absence of air, and the high conversion needs the reaction temperature to be as high as 190° C.
In U.S. Pat. No. 3,880,930, a sulfonic acid type cation exchange resin, benzene sulfonic acid or toluene sulfonic acid are used to catalyze the auto-condensation of cyclohexanone to produce dimers. The highest conversion is 59.9%, and the selectivity is 90%. However, the biggest disadvantage of an acidic resin is its poor heat resistance, and therefore the reaction must be carried out at a reduced pressure to lower the reaction temperature; otherwise, the catalyst tends to be deactivated, and the investment in equipment and the energy consumption are high. In addition, benzene sulfonic acid and toluene sulfonic acid are also strong organic acids, and therefore have the same disadvantages as sulfuric acid, which tends to corrode the equipment.
The research made by Institute of Coal Chemistry, Chinese Academy of Sciences, shows that the conversion of cyclohexanone is 52.4% and the selectivity of dimer is 99.2% when the dimerization of cyclohexanone is carried out in a packed bed reactor at 350° C. in the presence of 5% of NaNO2-rare earth element/γ-Al2O3 catalyst. However, the conversion is low, the reaction temperature is too high and the energy consumption is large, which make the process have no superiority [Fine Chemicals, 1994, 11(5), 42-45].
China Pat. No. 101 205 170 proposes an improved catalyst. The auto-condensation of cyclohexanone is carried out in a batch reactor with an Al2O3 catalyst obtained by calcination of a pseudo-boehmite precursor, and the yield can be 70% or above.